In a communication system in which adaptive modulation is performed, an optimal modulation scheme is selected based on propagation path characteristics that change with time. High-speed data communication can be performed by selecting the fastest modulation scheme that can satisfy the desired error rate (e.g., Packet Error Rate: PER=1%) based on propagation path characteristics. For example, when adaptive modulation is applied to downlink channels, the propagation path characteristics measured by a mobile station at the data receiving end are reported to a base station at the data transmitting end, and then the base station selects an optimal modulation scheme for the reported current propagation path characteristics and transmits data to the mobile station.
In the communication system in which such adaptive modulation is performed, the average signal to noise ratio (SNR) measured at the data receiving end is most commonly used as the value representing propagation path characteristics. Furthermore, in order to improve the accuracy of modulation scheme selection, a method of selecting a modulation scheme is also proposed taking into account delay spread as well as average SNR (for example, see H. Matsuoka, T. Ue, S. Sampei and N. Morinaga, “An Analysis on the Performance of Variable Symbol Rate and Modulation Level Adaptive Modulation System”, TECHNICAL REPORT OF IEICE, RCS 94-64 (1994-09), pp. 31-36: hereinafter referred to as “reference 1”). In addition, in multi-carrier communication system such as orthogonal frequency division multiplexing (OFDM) system, a method of selecting a modulation scheme is also proposed based on average SNR and variation in propagation path characteristics between adjacent subcarriers (for example, see Unexamined Japanese Patent Publication No. 2001-103032: hereinafter referred to as “reference 2”).
Now, when adaptive modulation is applied to a multi-carrier communication system, adaptive modulation is implemented per subcarrier. Therefore, at the data receiving end, it is necessary to report to the data transmitting end the value representing propagation path characteristics per subcarrier.
For example, in a mobile communications system in which frequency scheduling is performed such that the base station assigns to a plurality of mobile stations different subcarriers based on the propagation path characteristics of the downlink channel of each subcarrier, all of the plurality of mobile stations report to the base station the propagation path characteristics per subcarrier, and the volume of traffic increase on uplink channels. In order to solve this problem, it has been proposed to divide a plurality of subcarriers is into a number of blocks (i.e., block division of subcarriers) and carry our frequency scheduling on a per block basis. According to this method, since each mobile station has only to report propagation path characteristics on a per block basis, the volume of traffic on uplink channels can be reduced considerably compared with the case where propagation path characteristics are reported on a per subcarrier basis. If adaptive modulation is applied to a communication system in which such block division of subcarriers is carried out, all subcarriers belonging to the same block are modulated with the same modulation scheme.
However, in the above-noted prior art examples, if adaptive modulation is performed in a communication system where block division of subcarriers is carried out, there is a problem that the optimal modulation scheme cannot be accurately selected, for the following reasons.
For instance, since the delay spread in above reference 1 represents variations in propagation path characteristics over full bandwidth, it cannot represent the variation in narrowband propagation path characteristics of each block, when subcarriers are divided into blocks. Consequently, when subcarriers are divided into blocks, the optimal modulation scheme cannot be selected accurately.
One instance for estimating the variation in propagation path characteristics between adjacent subcarriers as in the above reference 2 based on SNR variation is shown in FIG. 8. Namely, in case a, the SNR value varies between 2 and 3 among four subcarriers in one block, and so the normalized SNR error representing the SNR variation between adjacent subcarriers is 0.3. On the other hand, in cases b and c, although the variation of SNR values among four subcarriers in one block is greater than in case a, the normalized SNR error is 0.3, which is the same as in case a. In this way, when subcarriers are divided into blocks, the variation in propagation path characteristics between adjacent subcarriers (i.e. normalized SNR error) sometimes have the same value both in case a where SNR variation is relatively small and in cases b and c where SNR variation is relatively large. Under such circumstances, the variation in propagation path characteristics with in each block cannot be estimated accurately, and the optimal modulation scheme cannot be selected accurately for cases a to c, when subcarriers are divided into blocks.
As mentioned above, when bock division of subcarriers is carried out, it is difficult to accurately select the optimal modulation by the method of reference 1 or reference 2 in cases where subcarriers are divided into blocks. Therefore, to perform adaptive modulation in communication systems in which block division of subcarriers is carried out, it is necessary to introduce new parameters that optimally represent variations in narrowband propagation path characteristics of each block.